Rotary blood pumps having magnetically supported or partially magnetically supported bearings have advantages including low wear, low thrombogenicity, and long durability. The inventors of prior art devices have generally sought to suspend a rotating impeller utilizing various types of sensors and feedback control methods similar to those used with general purpose magnetically suspended bearings. The objective of completely magnetically suspended blood pumps has been to provide wide clearance gaps between the rotating impeller and other parts of the device so that blood could flow through the gaps to avoid stagnation and clotting. Complete elimination of contact between rotating and stationary parts has the advantage of avoiding friction and heat generation which can damage blood. Olsen U.S. Pat. No. 4,688,998, and Moise U.S. Pat. No. 4,779,614 utilized sensors and servo-control systems of the general type described to obtain full magnetic suspension, however none of these or other fully magnetically suspended blood pumps has proved clinically successful due to their large size and complexity or to other unsolved problems.
Jarvik, U.S. Pat. No. 4,994,078, discloses blood pumps having blood immersed mechanical radial bearings and partial magnetic thrust bearings. High velocity washing of the junction of the rotating and stationary components of various embodiments of this invention has proven successful and several devices incorporating this principle have functioned successfully for many months in animals and one design has been successfully used in human patients. Jarvik U.S. Pat. No. 5,507,629, discloses blood pumps with radial magnetic bearing support and mechanical thrust bearing support, in which the mechanical bearing is also washed by high blood flow. Wampler, U.S. Pat. Nos. 5,840,070 and 5,695,471 disclose similar devices having partial magnetic bearing support together with mechanical thrust bearings. Like Jarvik, U.S. Pat. No. 5,507,629 the magnetic forces which support the rotor radially are exerted across relatively wide channels through which blood flows. The magnitude of the magnetic forces and size of the gaps across which they are exerted limits the stiffness of the radial bearing and the rotors of these devices are subject to either radial or axial displacement due to blood flow and pressure forces as well as shock, vibration and other inertial forces.
The present invention improves upon previous partially magnetically suspended blood pumps by minimizing the size of the magnetic components needed to achieve adequate radial stiffness. This permits miniaturization in the size of the overall pump which has important advantages, particularly when intraventricular placement is utilized (Jarvik, U.S. Pat. No. 5,092,879). Radial magnetic support is achieved by utilizing small very closely adjacent permanent magnets or a combination of permanent magnets and other non-magnetized ferromagnetic components. By this functional structure, the present invention permits stiffer radial bearings which has several important advantages regarding pump design and efficiency. The invention is distinguished from all prior magnetically suspended blood pumps because only a slight surface film of blood occupies the gap across which the suspending magnetic forces are exerted, and there is essentially no blood flow through this gap.
With "null position" magnetic bearings of the present invention the rotor is axially confined at each of its two ends by mechanical thrust bearings and is positioned extremely close to an axial position where there is no net magnetic axial force upon it in either direction. One embodiment of the invention includes means to precisely adjust the axial forces produced by the permanent magnets to locate the null position within a very tiny range of axial motion permitted by mechanical thrust bearings. The thrust bearings also stablize the rotor by preventing tilt with regard to the axis of rotation.